The present invention relates to an optical device for use in an image display device and, more particularly, to a small-sized light-weight optical device having a super-wide angle optical system and a head-mounted display provided with said optical device.
Recently, an increasing interest has been given to head-mounted display units as means for providing visual information in virtual reality and for realizing remote control of various kinds of robots. The essential requirements for such head-mounted display units are: they must have a wide-angle of visibility and high-resolution sufficient to provide visual information with reality and ambience and must be minimized in sized and weight enough to be easily worn on a part of body, particularly, on the head. The head-mounted display has a spectacle-like casing which is wearable on the head. The casing contains, in each of its lens-parts, a back-light, a liquid crystal panel (liquid crystal display) and a lens, which are arranged in the described order from the outside. The user can view an enlarged image on the liquid crystal panel in the said head-mounted display as though he looked at the image on a large screen.
With rapid progress in developing various liquid crystals in the past years, small-sized high-quality liquid crystal panels have been supplied for use in head-mounted display units. However, an optical device which is another main component of the head-mounted display has been insufficiently developed in respect to reducing its size and weight.
On the other hand, the head-mounted display units which have been used mainly hitherto in industrial applications and experimental applications of virtual reality, are now finding an increasing demand in home video-games and VTR display units. Accordingly, the saving in size and weight of head-mounted display units is further keenly required.
To solve the above-mentioned problems, an art has been proposed in the Japanese Laid-Open Patent Publication No. 6-59217. A head-mounted display has a liquid crystal display disposed at its top, wherefrom light L travels and is reflected backward by a semitransparent mirror, then is enlarged by a concave mirror and reflected back to the observer's eye. In the above case, the light is folded between the semitransparent mirror and the concave mirror, thereby the optical device used may have a saved size as compared with a linearly arranged device.
The prior art device with the liquid crystal panel arranged at the top, however, has the drawback that a reduction in size may be realized only with a reduced angle of visibility. Namely, to increase the angle of visibility for obtaining visual information with a high sense of immension, the semitransparent mirror having a larger size must be used in front of the liquid crystal panel, thereby the total thickness of the head-mounted display is increased and thus nullifies the size-saving effect of the optical device. In the above case, the angle of visibility from an observer is limited to not more than 90 degrees since the observer's eyes can not be advanced toward the concave mirror nearer than the semi-transparent mirror.
To solve the problem of saving in size and weight of a head-mounted display, a prior art device is disclosed in Japanese laying-open Patent Publication No. 1-133479. The head-mounted display has a liquid crystal display disposed at its top, wherefrom light L is deflected outwardly by a plane reflecting mirror, enlarged and reflected by a concave mirror and enters the eye. In this case, the light is folded between the plane reflecting mirror and the concave mirror, thereby the optical device used may have a saved size as compared with a linearly arranged device.
The prior art device with the liquid crystal panel arranged at the top, however, has the drawback that a reduction in size may be realized only with a reduced angle of visibility. Namely, to increase the angle of visibility for obtaining visual information with a high sense of immension, the semitransparent mirror having a larger size must be used in front of the liquid crystal panel, thereby the total thickness of the head-mounted display is increased and thus nullifies the size-saving effect of the optical device.
An optical device for use in a head-mounted display must be not only compact and light enough to be easily worn on the user's head but must have a wide angle of visibility and a short focal length to provide a vivid image to the observer. A prior art intended to solve the above-mentioned problem is disclosed in the Japanese Laying-Open Patent Publication No. 1-126620 and Journal "VR news" vol. 3 issue Mar. 2, 1994. This device, however, is poor in its technical performance. A display displays an image from which light rays pass through a circularly polarizing filter and a concave mirror with a half-mirror coating and then are reflected by a cholesteric liquid crystal element. The reflected light rays are enlarged and reflected again by the concave mirror and fall onto the cholesteric liquid crystal element through which the rays are transmitted to eyes of the observer.
In the above device, the light path is folded between the cholesteric liquid crystal element and the half-mirror coated concave mirror, thereby the device may have a reduced size as compared with a device using an linearly arranged optical system.
The above-mentioned prior art involves three problems mentioned below:
The first problem is that an image may have a large curvature because the image is magnified only by the concave mirror. When an image formed on a planar screen of a display is viewed through an optical system having a large curvature, said image may be focused at its center portion but be out-of-focus at its peripheral portion. On the other hand, the optical system for the head-mounted display must have a high magnifying power and a super-wide angle of visibility to create an attractive image with an increased sense of reality. The above-mentioned optical system may increase the degree to which the image is out-of-focus if it is used in the prior art device. In other words, the prior art is unsuitable for realizing the required head-mounted display.
The second problem is the impossibility of realizing an optical system which has a large diameter and a short focal length. This is because increasing the diameter of the optical system increases the thickness of the concave mirror to the extent that the focus is included in the concave mirror. The head-mounted display has to sufficiently enlarge an image to fill the visual field. Therefore, the prior art can not be applied for this purpose.
The third problem is that the optical system of the prior art has a low transmission factor (25% in an ideal case) that makes the image dark. To overcome this drawback, it is needed to increase the luminance of the display four times or more, causing other disadvantages such as to increasing the size of the device and the electric power consumption.